How would you design a distributed rate limiter?
⚡ Short Answer
Use a token-bucket or sliding-window-log/counter algorithm, keyed per client, with counters in a shared low-latency store (Redis) so the limit is enforced globally across all app instances. Update atomically (Lua/INCR+EXPIRE), and decide fail-open vs fail-closed if the store is down.
☕Coffee Chat Question
Concept Made Simple
“How would you design a distributed rate limiter?”
🧠Mind Map Answer
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🔥What If?
Think Beyond the Expected
Why not just keep the counter in each app instance's memory?
Per-instance counters let a client multiply the limit by the instance count and reset on deploys/scaling — the global limit isn't enforced. A shared atomic store (Redis) gives one consistent count across instances; the gateway is a common enforcement point.
😂Real World
Distributed rate limiting lives at the gateway with Redis-backed counters and atomic Lua scripts; the design questions are algorithm choice (burst vs smoothness) and fail-open/closed behavior.
🎯Interviewer's Expectation
Keywords they're listening for:
⚠️Common Mistakes
- ✗Per-instance counters (limit multiplied)
- ✗Non-atomic read-modify-write races
- ✗No plan for the limiter store failing
✅Best Practices
- ✓Shared atomic counters (Redis Lua)
- ✓Enforce at the gateway
- ✓Choose algorithm for burst vs smoothness; decide fail mode
🔁Follow-up Questions
- 1Token bucket vs sliding-window-log vs counter?
- 2Fail-open or fail-closed when Redis is down?
- 3How do you handle bursts vs sustained rate?
🧩Related Technologies
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Plain-language foundations
I'm preparing for a software engineering interview and want to understand this from scratch, as a beginner. Topic: Scaling (System Design) Interview question: "How would you design a distributed rate limiter?" Please: 1. Explain the core idea in simple, plain language, using an everyday analogy. 2. Define any technical terms you use. 3. Walk through one small, concrete example. 4. Finish with a single sentence I can easily remember. Keep the tone friendly and assume I'm new to this topic.
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